Virtual computer systems, in which more than one operating system runs on a computer system, have been known for decades. Variations of virtual computer systems include'architectures in which plural operating systems run on a single processor, plural operating systems run on plural processors, and plural operating systems run on plural processors that are connected by an input/output (I/O) bus such as a personal computer interconnect (PCI) bus.
Virtual computer systems include architectures in which one or more of the operating systems runs above a native operating system, and architectures in which plural operating systems run above a virtual machine monitor (VMM) or hypervisor layer. Such a VMM or hypervisor can provide a common platform for those operating systems that run above it, and a VMM or hypervisor layer may emulate hardware to the operating systems running above it.
As with other conventional computer systems, an operating system for a virtual computer system may contain a file system that organizes data stored on a disk or other storage system, and a network protocol stack for communicating, via a network interface device, with other entities over a network. When different operating systems of a virtual computer system wish to communicate with each other, for example to exchange data, they typically do so via networking protocols.
Certain networking protocols, such as Transmission Control Protocol (TCP), provide guaranteed delivery of data and other features that require significant computing resources to run. For example, TCP requires a complex control block, sometimes called a TCP control block or TCB, to be maintained at a network node such as a computer system for each logical connection that is set up to provide TCP services. Such a TCB contains status information that fully describes the logical connection from the standpoint of the node by which it is maintained, and so can also be called a TCP connection. An exemplary TCP control block is discussed and illustrated in chapter 24, pages 795-815 of “TCP/IP Illustrated, Volume 2,” Wright and Stevens (1994), which is incorporated herein by reference. Due to the resources required to run TCP, some network interfaces, whether provided as add-on-cards or board-level products such as chipsets, have processors or other hardware that offload processing of TCP from a central processing unit (CPU) of the computer system.
When different operating systems of a virtual computer system exchange data using TCP, the resources required of the virtual computer system to exchange the data are typically doubled in comparison with a computer system that runs only one of the operating systems communicating over a network. For example, to send-data by TCP, a first operating system of the virtual computer system establishes a TCP connection with a second operating system of the VC system, after which the first application may request to send data to the second application. The data is then acquired by the network stack of the first operating system and split into TCP/IP segments which are prefixed with TCP/IP headers including checksums of both the data and the headers, each step of which can include copying the data by the CPU of the VC. Each of the packets containing headers and data is then prefixed with a data link layer header and transmitted on the network, only to be received from the network by the same VC system, which essentially reverses the process performed by the first operating system and first protocol stack, in order to receive the data by the second operating system and second protocol stack. That is, the second protocol stack analyzes the headers and checksums of each received data packet, and reassembles the data from the packets, and then provides the data to the second application, each step of which can again include copying the data by the CPU of the VC.
Because the data to be sent from a first application running on a first operating system of a VC to a second application running on a second operating system of the VC may be stored on a memory that can be accessed by both operating systems, proposals have been made to transfer data between guest operating systems by memory mapping procedures instead of using network protocols such as TCP. While such memory remapping could eliminate much of the double copying described above, the logistics are complex and perhaps for this reason are not commonly implemented.